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Pheromone ‘image’ key to mouse’s mating

By Shaoni Bhattacharya, Denver

Mice use their sixth sense to form a pheromonal “image” of other mice, which is as detailed as the visual images of faces so important to humans and other primates.

For the first time, scientists were able to watch the brains of mice react as they sniffed the faces of other mice, to pick up the chemical cues. The pheromonal images reveal social status and how closely related the mice are, determining behaviours such as mating, fighting and mother-pup bonding.

The facial area is an abundant source of pheromones, says lead researcher Lawrence Katz, at Duke University, North Carolina. This might be why people enjoy kissing so much, he speculates, though it is unlikely that modern humans rely so heavily on pheromone clues.

“It’s quite wonderful,” Katz told the American Association for the Advancement of Science conference in Denver, Colorado. “Our main finding here opens up one of the remaining black boxes of the brain. It’s no less than whales communicating by song or bats by ultrasound.”

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Olfactory bulb

Many mammals can sense pheromones, but this is the first study to show the how detecting pheromones can build up a complex image of an individual.

Katz and colleagues used highly specific microelectrodes to study the response of individual nerve cells in the brains of individual mice when they were placed close to another.

The team looked at the response of about 50 nerve cells in a specialised part of the mouse brain called the accessory olfactory bulb, which deals with pheromonal information. The cells receive signals of pheromone detection from a special organ, called the vomeronasal organ (VNO), at the back of a mouse’s nose.

As well as proving that this area of the brain was able to construct a pheromonal picture, the team also found that the nerve cells only fired when animals were in close physical contact. This helps explain why some mammals sniff each other so closely.

Cheek to cheek

“For many years it was known that urine was a very potent source of pheromones for animals. What has been much less widely appreciated is that the facial area is an abundant source of pheromones,” says Katz.

There may be some echo in human behaviour, he says&colon; “In kissing, we are probably reflecting behaviour necessary to receive sufficient concentrations of those pheromones.”

However, there is no consensus on whether humans have a VNO or not. And Katz notes that rodents have 300 genes devoted to smells detected by the VNO. Humans have only a couple – the rest have turned into defunct genes known as “pseudogenes”. These genes may have decayed as primates increasingly relied on their well-developed visual senses to assess potential partners.